Transplacental carconogenic studies in the Old World monkey, Erythrocebus patas, have shown that pregnant females tolerate, without interruption of pregnancy, at least 6 successive biweekly intravenous injections of 1-ethyl-1-nitrosourea (ENU) each of 0.1 mmole ENU/kg body weight, for a total weight-adjusted dose which is more than that necessary to induce a 100 percent incidence of nerve tumors in rats. Efforts to increase this dosage continue. No tumors of any kind have been observed in monkeys exposed to this regimen in utero and observed up to 24 months after birth. On the basis of this data, it seems unlikely that ENU will induce a high incidence of the pediatric types of human neoplasms appearing early in postnatal life. To exclude the possibility that a much different proportion of the dose of ENU given the mother reaches the fetal monkey compared with the fetal rat. Current efforts are directed toward study of pharmacodynamics of maternal/fetal distribution, tissue binding, and release ENU-ethyl-C14, and investigation of the relative doses required for acute (teratogenic) damage to the CNS in the rat and monkey during organogenesis.